Effort has been expended in the past on reducing the noise emitted from a duct or, e.g., at the end of a muffler pipe, by the provision of a flow modulator therein. This has been studied in an effort to find an alternative solution to "actively" creating anti-noise. The flow modulator can actively control the process leading to strong low frequency emission at the exhaust of pulsed flows.
The type of flows found in the exhaust systems of internal combustion engines or compressors we are interested in usually have an average flow velocity ranging from 20 to 60 m/s. The fundamental frequencies are very low (below 100 Hz). Due to the quasi-incompressibility of the fluid at this low Mach number, the pressure fluctuations are almost in phase everywhere in straight ducts and, strictly speaking, do not "acoustically" propagate. The exhaust mouth of incompressible pulsed flows can be identified with an acoustic monopole (of diameter r.sub.0). At a distance R from this source, the acoustic pressure (P(R,t)) is proportional to the variation in time in the mass flow (Q.sub.m (t-R/c)): ##EQU1##
The acoustic energy radiated by a pulsed flow is related mainly to the pulsatory character of the flow. This causal mechanism between Fluid Mechanics and Acoustics shows that reducing the flow modulation amplitude should attenuate the low frequency sound level. However, the existence of walls and the turbulence of the flow constitute "dipolar" and "quadripolar" acoustic sources. Because of their poor efficiency, they will produce a background large band noise, below which the discrete spectrum lines linked to the pulsation cannot be attenuated.
The regulation of a flow is usually provided by a device which modifies the "head loss" (e.g., a valve). When the flow fluctuates periodically around a fixed mean value, the solution is to provide a unit which controls that modulation in real time.
Such a system has been disclosed by L. Hardouin, J. Tartarin and J. Laumonier in DSC Vol. 38, Active Control of Noise and Vibration, ASME, 1992 and in Laboratoire d'Etudes Aerodynamiques (Unite de Recherches Associee au C.N.R.S., No. 191, Poitiers, France) and VPI conference on Active Noise Control, 1991, pp 150-162. This system is diagrammed in FIG. 1. The flow modulator device is a circular duct containing a circular disk which can rotate about a shaft. The shaft is driven by a stepper motor. Because the motor will be rotating the flap at a high frequency, on the order of 100 cycles/second, the shaft bearing assembly must be designed with very tight tolerances. It must be free to rotate but it must not be prone to rattling or any kind of non-rotational motion. The inertia of the flap and the high cyclic rate require a powerful stepper motor. This point is mentioned by Tartarin while commenting on one of his early efforts that did not work.
Additional work using devices of this kind is described in Automotive Engineering for February 1993, pp 13-16. It is shown in this paper that active attenuation of pressure pulsations in engine exhaust flow can achieve significant sound pressure reductions. Motor-driven oscillating and rotating valves are used. However, the rotating valve (that which makes one complete revolution per noise cycle) is restricted to operating at one frequency only. In automotive applications, the issues of size, weight, and cost are especially significant concerns.
The instant invention is an improvement over the background art inasmuch as it provides a unique modulator arrangement which eliminates the use of having to physically move the modulator by a stepping motor or the like. It uses piezo actuators on the modulator plate to move it.
The advantages of the piezo flow modulator are:
Low complexity. There are essentially no moving parts. Only a power amplifier is required to execute the controller's command. This is in contrast to a stepper motor, motor controller, and bearing assembly as required by Tartarin. PA1 Low cost. Accountings version of the item above. PA1 Low power consumption. The power consumed will be mainly due to heating the amplifier and should be lower than motor based designs.
Accordingly, it is an object of this invention to provide an improved flow modulator for reducing duct noise.
It is another object of this invention to provide a flow modulator operated by electrically stimulating piezo actuators.